Chapter 1 Discussion Questions
5. Defi ning a new object class that fully integrates and responds to its context
2.3 BEYOND PARAMETRIC SHAPES
In this section, we go a bit deeper into the features of parametric modeling–
based BIM systems, focusing on issues that extend beyond pure parametric geometric modeling.
2.3.1 Relational Structures
When we place a wall in a parametric model of a building, we can associate the wall to its bounding surfaces, its base fl oor planes, the walls its ends abut, any walls butting it, and the ceiling surfaces trimming its height. It also bounds the spaces on its two sides. These are all relations in the parametric structure that are then used to manage updates. When we put a window or door in the wall, we are defi ning another type of relation between the window and the wall (and also the spaces on both sides). Similarly, in pipe runs, it is important to defi ne whether connections are threaded, butt-welded, or have fl anges and bolts.
Connections in mathematics are called topology and—distinct from geometry—
are critical to the representation of a building model and are one of the funda- mental defi nitions embedded in parametric modeling.
Other kinds of relations are also fundamental to parametric layouts.
Reinforcing is contained in the concrete in which it is a part. Framing is part of a wall. Furniture is contained in a space object. Aggregation is the general term for “part of” relationships. It is a generalized relation that is used for access- ing objects and is managed either automatically or manually in all BIM design systems. Aggregation is used for grouping spaces into departments, parts into assemblies, pieces into part orders, and pieces into erection sequences, for example. Rules can be associated with aggregations; how the assembly proper- ties are derived from the part properties, for example.
ch002.indd Sec3:57
ch002.indd Sec3:57 3/8/11 8:17:05 AM3/8/11 8:17:05 AM
www.EngineeringBooksPdf.com
Relations carry three important kinds of information: what can be con- nected or the parts of an aggregation; some relations have one or more fea- tures, such as how a connection modifi es the parts to which it is connected;
and last, the properties of the relation.
Relations are critical aspects of a BIM model specifi cation that determines what kinds of rules can be defi ned between parts. They are also important as design objects and often require specifi cation or detailing. In none of the BIM design tools is an explicit defi nition of the relations allowed and not allowed. They may be identifi ed in an ad hoc manner embedded in documen- tation. Thus users will have to sort these out themselves. In architectural BIM design applications, connections are seldom defi ned as explicit elements. In fabrication-level BIM design applications, they are almost always explicit ele- ments. To our knowledge, there has not been a careful study of the topological relations that should be supported in BIM applications.
2.3.2 Property and Attribute Handling
Object-based parametric modeling addresses geometry and topology, but objects also need to carry a variety of properties if they are to be interpreted, analyzed, priced, and procured by other applications.
Properties come into play at different stages in the building lifecycle.
For example, design properties address space and area names, properties for spaces such as occupancy, activities, and equipment performance needed for energy analysis. Zones (an aggregation of spaces) are defi ned with properties dealing with thermal controls and loads. Different system elements have their own properties, for structural, thermal, mechanical, electrical, and plumbing behaviors. Later, properties also address materials and quality specifi cations for purchasing. At the fabrication stage, material specifi cations may be refi ned to include bolt and weld and other connection specifi cations. At the end of construction, properties provide information and links to pass operating and maintenance data onto operations and maintenance.
BIM provides the environment to manage and integrate these properties over the project lifecycle. However, the tools to create and manage them are only starting to be developed and integrated into BIM environments.
Properties are seldom used singularly. A lighting application requires mate- rial color, a refl ection coeffi cient, a specular refl ection exponent, and possibly a texture and bump map. For accurate energy analysis, a wall requires a different set. Thus, properties are appropriately organized into sets and asso- ciated with a certain function. Libraries of property sets for different objects and materials are an integral part of a well-developed BIM environment. The property sets are not always available from the product vendor and often have
ch002.indd Sec3:58
ch002.indd Sec3:58 3/8/11 8:17:06 AM3/8/11 8:17:06 AM
www.EngineeringBooksPdf.com
to be approximated by a user, the user’s fi rm, or from the American Society of Testing and Materials data (ASTM). Although organizations such as the Construction Specifi cations Institute are addressing these issues (see Section 3.4.1 and 3.4.2), the development of property sets for supporting a wide range of simulation and analysis tools have not yet been adequately organized in a standard way for use; currently, they are left to users to set up.
Even seemingly simple properties can be complex. Take space names; they are used in spatial program assessment, functional analysis, and sometimes for early cost estimation and assigning energy loads and their schedules of use. Space names are building type–specifi c. Some organizations have tried to develop space name standards to facilitate automation. GSA has three dif- ferent space name classifi cations for court houses: for building type spatial validation, another for lease calculations, and yet another set used in the U.S.
Courts Design Guide. At both the department and individual space levels, Georgia Tech estimated there are about 445 different valid space names (Lee et al. 2010).
Current BIM platforms default to a minimal set of properties for most objects and provide the capability of extending the set. Users or an applica- tion must add properties to each relevant object to produce a certain type of simulation, cost estimate, or analysis and also must manage their appropriate- ness for various tasks. The management of property sets becomes problematic because different applications for the same function may require somewhat different properties and units, such as for energy and lighting.
At least three different ways exist that properties may be managed for a set of applications:
By predefi ning them in the object libraries so they are added to the design model when an object instance is created
By the user adding them as-needed for an application from a stored library of property sets
By the properties being assigned automatically from a database as they are exported to an analysis or simulation application, based on an index or key
The fi rst alternative is good for production work involving a standard set of construction types but requires careful user defi nition for custom objects.
Each object carries extensive property data for all relevant applications, only some of which may actually be used in a given project. Extra defi nitions may slow down an application’s performance and enlarge a project model’s size.
The second alternative allows users to select a set of similar objects or property
•
•
•
ch002.indd Sec3:59
ch002.indd Sec3:59 3/8/11 8:17:06 AM3/8/11 8:17:06 AM
www.EngineeringBooksPdf.com
sets to export to an application. This results in a time-consuming export proc- ess. Iterated use of simulation tools may require the addition of properties each time the application is run. This would be required, for example, to examine alternative window and wall systems for energy effi ciency. The third approach keeps the design application light but requires the development of a compre- hensive material tagging system that can be used by all exporting translators to associate a property set for each object. The authors believe that this third approach is the desired long-term “solution” for property handling. The neces- sary global object classifi cations and name tagging required of this approach must still be developed. Currently, multiple object tags must be developed, one for each application.
The development of object property sets and appropriate object classifi ca- tion libraries to support different types of applications is a broad issue under consideration by the Construction Specifi cation Institute of North America and by other national specifi cation organizations. It is reviewed in more detail in Section 3.4.1 and 3.4.2.
Building Object Model (BOM) libraries, representing both objects and properties of specifi c commercial building products, are a potentially impor- tant part of a BIM environment for managing object properties. This type of facility is reviewed in Chapter 5, Section 5.4.
2.3.3 Drawing Generation
Even though a building model has the full geometric layout of a building and its systems—and the objects have properties and, potentially, specifi cations and can carry much more information than drawings —drawings will continue to be required as reports extracted from or as specialized views of the model, for some time into the future. Existing contractual processes and work culture, while changing, are still centered on drawings, whether paper or electronic. If a BIM tool does not support effective drawing extraction and a user has to do signifi cant manual editing to generate each set of drawings from cut sections, the benefi ts of BIM are signifi cantly reduced.
With building information modeling, each building object instance—its shape, properties, and placement in the model—is represented only once.
Based on an arrangement of building object instances, all drawings, reports, and datasets can be extracted. Because of this nonredundant building represen- tation, all drawings, reports, and analysis datasets are consistent if taken from the same version of the building model. This capability alone resolves a signifi - cant source of errors. With normal 2D architectural drawings, any change or edit must be manually transferred to multiple drawing views by the designer, resulting in potential human errors from not updating all drawings correctly. In
ch002.indd Sec3:60
ch002.indd Sec3:60 3/8/11 8:17:06 AM3/8/11 8:17:06 AM
www.EngineeringBooksPdf.com
precast concrete construction, this 2D practice has been shown to cause errors costing approximately 1 percent of construction cost (Sacks et al. 2003).
Architectural drawings do not rely on orthographic projections, as learned in high school drafting classes. Rather, building plans, sections, and elevations incorporate complex sets of conventions for recording design information graphically on sheets of paper that vary for different systems. This includes symbolic depiction of some physical objects, dotted representation of geom- etry behind the section plane in fl oor plans, and very selective dotted-line representation of hidden objects in front of the section plane, in addition to line-weights and annotations. Mechanical, electrical, and plumbing systems (MEP) are often laid out in different ways in different stages of design. These different conventions require BIM design applications to embed a strong set of formatting rules in their drawing extraction capabilities. In addition, indi- vidual fi rms often have their own drawing conventions that must be added to the built-in tool conventions. These issues affect both how the model is defi ned within the tool and how the tool is set up for drawing extraction.
Part of a given drawing defi nition is derived from the object defi nition.
The object has an associated name, annotation, and in some cases view prop- erties with line weights and formats for presentation that are carried in the object library. The placement of the object also has implications. If the object is placed relative to a grid intersection or wall end, that is how its placement will be dimensioned in the drawing. If the object is parametrically defi ned relative to other objects, such as the length of a beam placed to span between variably placed supports, then the drawing generator will not automatically dimension the length unless the system is told to derive the beam length at drawing gen- eration time. Some systems store and place associated annotations with object sections, though these annotations often need shifting to achieve a well- composed layout. Other annotations refer to details as a whole, such as name, scale, and other general notes and these must be associated with the overall detail. Drawing sheets also include a site plan, which shows the building’s placement on the ground plot relative to recorded geospatial datum. Some BIM design applications have well-developed site-planning capabilities, others do not. Table 2–1 shows which BIM design applications include site objects.
Current BIM design tool capabilities come close to automated drawing extrac- tion, but it is unlikely that automation ever will be 100 percent complete.
Most buildings involve thousands of objects, from girders and foundation pads to baseboards and nails. It is usually thought that some types of objects are not worth modeling. They must still be depicted in the drawings for cor- rect construction, however. BIM design tools provide the means for extracting a drawn section at the level of detail to which they are defi ned in the 3D model
ch002.indd Sec3:61
ch002.indd Sec3:61 3/8/11 8:17:07 AM3/8/11 8:17:07 AM
www.EngineeringBooksPdf.com
(with certain objects selectively turned off). The location of the drawn section is automatically recorded with a section-cut symbol on a plan or elevation as a cross-reference and the location can be moved if needed. The section is then detailed manually with the needed wood-blocks, extrusions, silicon bead- ing, and weather stripping; and associated annotations provided in the fully detailed drawn section. An example is shown in Figure 2–13, with the fi gure on the left showing the extracted section and the one on the right showing the detailed section with drafted annotation. In most systems, this detail is associ- ated with the section cut it was based on. When 3D elements in the section change, they update automatically in the section but the hand-drawn details must be manually updated.
To produce drawings, each plan, section, and elevation is separately com- posed based on the above rules from a combination of cut 3D sections and aligned 2D drawn sections. They are then grouped into sheets with normal borders and title sheets. The sheet layouts are maintained across sessions and are part of the overall project data.
Drawing generation from a detailed 3D model has gone through a series of refi nements to make it effi cient and easy. Below is an ordered list of the levels of quality that can now be supported technically, though most systems have not realized the top level of capability for drawing generation. We start from the weakest level.
1. A weak level of drawing production provides for the generation of